Cytologic and immunohistochemical features seen in IDCH. (A) A skin biopsy with characteristic ETV3::NCOA2 fusion showing cytologic features of LCH. However, unlike LCH, admixed eosinophils are absent. (B-C) Skin and lymph node biopsies from the same patient with primary IDCH, which harbors TET2 and KRAS mutations. Variation in cytologic features even in the same patient is noted in different sites. (D) Skin biopsy from a patients with secondary IDCH, who has underlying chronic myelomonocytic leukemia (CMML), showing mostly similar features seen in panels A,C with a relatively higher nucleus-to-cytoplasm ratio. (E) Lymph node biopsy from 1 of the excluded patients who was detected to have a KMT2A::MLLT1 fusion showing somewhat similar morphologic features seen in panel D. (F) Skin biopsy of a patient with primary IDCH showing epidermotropism, which was occasionally present in IDCH. (G-J) Representative case with ETV3::NCO2 fusion: lesional cells are diffusely positive for (G) CD1a and (H) BDCA1/CD1c; whereas negative for (I) Langerin/CD207 and (J) CSF1R/CD115.